Enclosed rotor waterjet propulsors are gaining more acceptance yearly in the form of small 50-150 HP units for personnel watercraft and in mid-size 1,000-7,000 HP units for patrol craft, high speed passenger ferries, and some motor yachts. Even though grossly inefficient in small sizes, they are necessary for personnel watercraft from a safety standpoint when compared to exposed propellers. The mid-size units are mainly applied to vessels such as high speed passenger ferries that spend most of their time cruising at high speeds where the waterjets are relatively efficient. These waterjets are noted to be inefficient at low and mid-range speeds and they have speed and power operational limits imposed to reduce cavitation damage to their rotors. However, due to their benefits of shallow draft, low underwater noise signature, reduced maintenance compared to exposed propeller, shaft, and rudder propeller systems, more constant and smooth engine loadings, and reduced passenger cabin noise and vibration they are becoming a preferred selection for propulsion of high speed passenger ferries.
There are difficulties in applying waterjets to any air cushioned craft such as the Surface Effect Ship (SES) or Applicant's SEACOASTER air cushioned craft designs. That is because the air layer under air cushion craft is ingested into the waterjet's inlet and causes a severe degradation of waterjet performance. The reason for this deficiency is that the standard waterjet is a pressurized water system with a pressure building nozzle positioned downstream of the rotor or impeller. Even small amounts of air entrained or mixed in the water, five percent or less was a threshold noted in some tests, can cause a severe loss of impeller efficiency. Waterjet air ingestion problems are also evident on many standard planing hull craft when operating in rough water where the waterjet inlet can be broached.
Tests run by Pratt & Whitney Aircraft in 1967-1969 on a 3,200 horsepower waterjet demonstrated that the air ingestion problem existed and was severe with no easy solution. Tests on two of Applicant's waterjet propelled air cushioned passenger ferry designs, 350 passenger 38 knot 109 foot air cushioned vessels, at Avondale Industries, New Orleans, showed a severe degradation of performance when air ingestion into the twin 2,000 horsepower KaMeWa waterjets occurred. The inlets were modified on one of these vessels to drop down approximately 20 inches below the hull in a streamlined airfoil shape. This reduced but did not eliminate the air ingestion problem but at the cost of a very noticeable speed reduction. In summary, standard commercially available waterjet propulsors have severe limitations on performance imposed by cavitaion at low to mid-range speeds and rotor overspeed problems due to inlet air ingestion when operating at high speeds in rough seas. Further, their performance at low and mid-range speeds is generally considered to be poor. Applicant considers low speeds as 0 to 7 knots, mid-range speeds as 7 to 20 knots and high speeds as above 20 knots; however, for purposes of this application, high speed is defined as any marine vehicle speed of 15 knots or more.
Applicant's new marine propulsor, preferably called the Hydro-Air Drive or simply by its acronym HAD offers a rotor that, in its optimum running condition, runs with only about the lower one half receiving water flow. It has, in its preferred embodiment, an open discharge with no flow restricting pressure building nozzle downstream of its rotor. It avoids cavitation and is immune to the air or gas ingestion problems that plague standard waterjets. It is also possible to cancel the gas flow to Applicant's rotor at low to mid-range speeds and thus double the mass flow in the preferred embodiment. This results in a much higher thrust at those low to mid-range speeds than that possible with the standard waterjet with its relatively small controlled flow discharge nozzle. For a more detailed discussion of the Hydro-Air Drive please refer to Applicant's U.S. Pat. No. 5,505,639.
A 22 inch diameter Hydro-Air Drive has been built and has undergoing sea trials in a 40 foot V-hull boat. It is driven by a 400 horsepower Caterpillar diesel engine. Initial tests now underway at Ft. Lauderdale, Fla., indicate that mid-range thrust values are superior to commercial high speed waterjets. At speeds above 30 knots, performance also appears better than commercial waterjets. There were no signs of cavitation damage and no apparent operational difficulties due to inlet aeration even when operating at high speeds in rough seas.
First tests were conducted with the flow directing structure stopping substantially forward of the rotor. This resulted in sheets of water spray, generated due to the close proximity of the inlet lip to the water surface, impacting the rotor above the shaft centerline. Water directing structure in the form of a plate that terminated proximal the horizontal centerline of the rotor and approximately one fourth of a rotor diameter forward of the rotor was then installed. Results were outstanding and a speed increase of approximately seven knots resulted. Based on these results, Applicant notes that there is a definite defining distance between the termination of the inlet liquid directing structure and the rotor vanes. Applicant defines this distance as no more than one and one half rotor diameters with less than one half rotor diameter preferred.
Applicant also notes that gas flow to an upper portion of the rotor gives good results and that ambient air and/or engine exhaust or other gas supply means can be used as the gas. However, it is also possible to have inlet water flow directing structure terminate upstream and very close to the rotor and with no gas flow supplied to the upper portion of the rotor. In such instance the forward upper portion of the rotor vanes are essentially operating in a partial vacuum.
The use of an inlet valve assembly to direct liquid/gas flow to the rotor was presented in Applicant's issued U.S. Pat. No. 5,505,639; however, the instant invention adds refinements to that concept. It is noted that addition of a straight section as part of yet aft of the normally circular arc shaped valve mechanism improves accuracy of liquid flow to the rotor. It is sometimes desirable to add a valve, such as a butterfly or gate valve, upstream of the flow directing valve to insure a positive stoppage of gas flow to the rotor when such is desired such as when operating at low boat speeds or reversing.
A further refinement is that the flow directing valve can be made up of two or more separate and separately controllable sections. This feature allows the level of water supplied to the port side of the rotor to be different than the level of water supplied to the starboard side of the rotor. The advantage of this is that any rotor torque effects can be adjusted by varying water levels to its port and starboard sides. Also, though not tested as of this writing, it may be possible to improve overall rotor efficiencies with such an approach.
It is also to be noted that a flow directing structure can be used to direct inlet water only and that no gas flow need be supplied to the upper portion of the rotor. In such case, the upper portion of the rotor vanes would be operating in a partial vacuum at high marine vehicle speeds.
A new simple steering and reversing mechanism for marine propulsors such as waterjets and the instant invention that requires minimum actuation force is presented herein. This system uses a reverse steering guide vane assembly or nozzle positioned below the rotor discharge that is connected to and rotates at the same rate as a steering rudder. There is no reversing effect until a flow blocking discharge assembly or reversing bucket is lowered aft of the steering and reversing nozzle. This differs distinctly from German Patent 2217171 who offers a rudder that is independent of and separated from a set of 360 degree rotatable steering louvers. Flow blockage in the German Patent is accomplished by turning the steering rudder 90 degrees to the discharge flow thereby blocking the discharge from going rearward and redirecting it to the rotatable steering louvers. Both the rudder and the 360 degree rotatable steering louvers are independently driven which is not the case of the instant invention's simple substantially one piece unit that is driven by a common actuator. Since the instant invention's rudder, by working requirement, does not turn 90 degrees to the flow it does not require the high actuation forces of the referenced German Patent. Due to the aforementioned noteworthy distinctions there is little resemblance between the instant invention's steering and reversing mechanism and German Patent 2217171. Further, the instant invention offers an optional water deflecting mechanism, normally in the form of a flap like device, that can be positioned under its reversing guide vane assembly. This water deflecting mechanism keeps water from hitting the guide vanes during ahead operation and is simply pushed out of the way by the reversing discharge water flow during reverse operation.